Quantum key distribution (QKD) technology has been known in which an encryption key is shared safely between a transmitting device and a receiving device that are coupled to each other with an optical fiber using a single photon transmitted continuously between the transmitting device and the receiving device. It is guaranteed that the encryption key shared by the quantum key distribution technology is not sniffed on the basis of the principle of quantum mechanics. The shared encryption key is used for encrypted communication such as one-time pad encrypted communication. In the one-time pad encrypted communication, an encryption key having the same size (quantity) as transmitted and received data is used and the used encryption key is destroyed. It is guaranteed that encrypted data transmitted and received by the one-time pad encrypted communication cannot be decrypted by an eavesdropper who has unlimited knowledge on the basis of information theory.
It is, however, difficult for the conventional technology to increase a processing speed of sifting processing performed in quantum key distribution processing.